Vapor cycle system with de-superheater

ABSTRACT

A vapor cycle system comprises a compressor for compressing a coolant to form a superheated vapor, a de-superheater for cooling the superheated vapor to form a reduced temperature vapor by exchanging heat with a cooling fluid flow, a condenser for condensing the reduced temperature vapor to form a condensed liquid by exchanging heat with the cooling fluid flow, and an evaporator for evaporating the condensed liquid. The de-superheater is located downstream of the condenser in the cooling fluid flow, and a temperature of the cooling fluid flow is higher at the de-superheater than at the condenser.

BACKGROUND

This invention relates generally to thermal management, and particularlyto vapor cycle systems. In particular, the invention concerns thermalmanagement for an aircraft-based vapor cycle system.

Modern commercial aircraft typically include a number of differentheating and cooling systems for the cabin and cargo bay areas, galleyfacilities, power electronics, and avionics and radar systems. Each ofthese components has different thermal requirements and powerconstraints, making overall efficiency an important design criterion.

Most aircraft cooling systems utilize at least one vapor cycle system orVCS unit. The vapor cycle system includes a compressor for compressingthe coolant, and a condenser for condensing the compressed fluid, withheat dispersed to different cooling fluid streams. The coolant thenflows through an expansion valve to an evaporator, where the fluidexpands and cools.

In some configurations, cold VCS fluid is cycled through an AC pack forcooling cabin air. Alternatively, a number of independent cooling loopscan be used to cycle specialized coolants to heat loads distributedthroughout the aircraft, cooling the coolant by exchanging heat with thevapor cycle system at the evaporator.

In either configuration, weight and efficiency are always at a premium.This makes thermal management an important design consideration, withparticular respect to increasing efficiency, reducing the overall weightand size envelope, and maintaining system reliability and service life.

SUMMARY

A vapor cycle system comprises a compressor, a de-superheater, acondenser, and an evaporator. The compressor compresses a coolant toform a superheated vapor, and the de-superheater cools the superheatedvapor by heat exchange with a cooling fluid flow, forming a reducedtemperature vapor. The condenser condenses the reduced temperature vaporby exchanging additional heat with a cooling fluid flow, forming acondensed liquid.

The condensed liquid is expanded then evaporated in the evaporator,absorbing thermal energy and starting the cycle again. Thede-superheater is located downstream of the condenser in the coolingfluid flow, so that the cooling fluid temperature is higher at thede-superheater than at the condenser.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a thermal management system for anaircraft, with a vapor cycle system having a de-superheater.

FIG. 2 is a schematic illustration of the vapor cycle system andde-superheater, in an air-cooled embodiment.

FIG. 3 is a schematic illustration of the vapor cycle system andde-superheater, in a fuel-cooled embodiment.

DETAILED DESCRIPTION

FIG. 1 is a schematic illustration of thermal management system 10 foran aircraft. System 10 includes vapor cycle system (VCS) 12 withde-superheater 14, cooling system 16, fuel circulation system 18 withfuel cooler 20 (dashed lines), and integrated power unit (IPU) 22.Integrated power unit 22 generates electrical power and regeneratescooling air flow (dotted lines) for thermal management system 10.De-superheater 14 reduces losses in vapor cycle system 12 (solid lines),raising the coefficient of performance (COP) for thermal transfer tocooling system 16 (dot-dashed lines), and increasing the overallefficiency of thermal management system 10.

Vapor cycle system 12 includes de-superheater 14, compressor 24,condenser 26, economizer 28 and evaporator 30. In one embodiment, vaporcycle system 12 operates on a two-phase coolant or refrigerant fluidsuch as 1,1,1,2-tetrafluoroethane or R-134a (hydrofluorocarbonHFC-134a). Vapor cycle system 12 is also operable on older refrigerantssuch as R-12 (chlorofluorocarbon CFC-12) or R-22(hydrochlorofluorocarbon HCFC-22), but in modern applications “green”fluids are typically used, including R-134a and other HFC, haloalkaneand halocarbon-based refrigerant fluids with relatively shortenvironmental lifetimes and reduced potential for ozone depletion.

The VCS loop is driven by compressor 24, which compresses therefrigerant to a superheated phase. The superheated phase is a gaseousor vapor state, at a temperature and pressure above the saturation andcondensation points. De-superheater 14 cools the superheated vapor byexchanging heat with the cooling fluid stream, “de-superheating” thefluid to improve efficiency or decrease weight for the vapor/liquidphase transition in condenser 26.

In particular, de-superheater 14 improves the performance of condenser26 by delivering fluid in a cooler vapor state, as compared to thesuperheated vapor output of compressor 24. Cooler vapor condenses morequickly, because less heat must be removed to reach the condensationtemperature, increasing the efficiency of vapor cycle system 12.

De-superheater 14 also utilizes a higher-temperature (hotter) flow,downstream of condenser 26 and fuel cooler 20, while condenser 26 use acooler fluid flow, upstream of fuel cooler 20 and de-superheater 14.Because the de-superheat temperature is higher than the condensationtemperature, temperature differential ΔT is less at both de-superheater14 and condenser 26. This reduces the change in entropy and improves thecoefficient of performance, as described below.

Refrigerant exits condenser 26 in a two-phase (liquid/vapor) state, atapproximately the condensation temperature. Upstream expansion valve 32Aprovides a minor expansion into economizer 28, further reducing thetemperature before entering evaporator 30 through downstream (major)expansion valve 32B.

The refrigerant fluid enters evaporator 30 as a sub-cooled liquid orcompressed fluid, or in a two-phase liquid/vapor state. The refrigerantevaporates and expands in evaporator 30 to produce a gas or vapor phase,absorbing heat from cooling system 16 as described below.

Fluid from evaporator 30 enters compressor 24 in a substantially gaseousor vapor state. In two-stage compressor embodiments, as shown in FIG. 1,economizer 28 also delivers cooled fluid to mixer 34, which reduces thetemperature of fluid exiting first compressor stage C1. This improvesheat transfer for cooling motor M, and eases second-stage compression byreducing the energy required to compress the refrigerant to asuperheated vapor state in second compressor stage C2. Along withde-superheater 14, these additional VCS components further increase thecoefficient of performance by reducing temperature differentials, andlowering the total entropy of vapor cycle system 12.

Cooling system 16 absorbs heat from load 36, and disperses the heatthrough thermal transfer to evaporator 30 of vapor cycle system 12. Insome cooling systems 16, heat is also dispersed to the cooling air flowvia heat exchanger (HX) 38.

Heat load 36 includes one or more aircraft systems that require heatingor cooling, for example a cabin, cockpit, cargo bay or galley chiller,or electronics components for radar, weapons control, avionics andcabin-based electronics or display systems. In power electronics coolingsystem (PECS) embodiments, heat load 36 may include power electronicsfor flight control actuators and other flight-critical systems.

Cooling system (or cooling loop) 16 operates on a refrigerant fluid withhigh heat transfer capability over a wide temperature range, for examplepolyalphaolefin (PAO) or hydrogenated PAO. Alternatively, cooling system16 operates on a silicate ester or oil-based coolant fluid such asCoolanol®, as available from Exxon Mobile of Fairfax, Va., or anotherfluid such as water, glycol, etc.

The relative flows of cooling system refrigerant through heat exchanger38 and evaporator 30 are controlled via bypass valves 40A and 40B, basedon cooling demands, ambient temperature, flight conditions and theavailable cooling air flow. For example, bypass valves 40A and 40B canbe adjusted to regulate higher levels of air cooling in heat exchanger38 during cruise flight conditions, and higher levels of evaporatorcooling in evaporator 30 during takeoff and landing, or during groundoperations.

Fuel circulation system 18 comprises fuel-air cooler 20, fuel tank 42,return-to tank (RTT) cooler 44, fuel-oil cooler (FOC) 46, and secondary(fuel-air) heat exchanger 48. Fuel system 18 typically operates on akerosene-type jet fuel such as Jet A or Jet A-1, or a naphtha-type fuelsuch as Jet B for low-temperature performance. In military applications,fuel system 18 operates on a modified kerosene-based fuel such as JP-5or JP-8, or a modified naphtha or “wide-cut” fuel such as JP-4.

Heat transfer in fuel circulation system 18 is determined according tothe temperature requirements of the various fuel subsystems, and basedon the different performance demands and fuel system capabilities ofmilitary-type aircraft, as compared to commercial designs. In theparticular embodiment of FIG. 1, for example, flow from fuel tank 42circulates through fuel cooler 20 to reduce downstream operatingtemperatures, with fuel cooler 20 located between condenser 26 andde-superheater 14 in the cooling fluid flow. The fuel-air exchangetemperature at fuel cooler 20 is thus higher than the condensationtemperature at condenser 26, and lower than the de-superheat temperatureat de-superheater 14.

On afterburning turbofan engines, boost pump 50 supplies fuel to inletvalve 52 for afterburner (AB assembly) 54, in order to provide thrustaugmentation during short periods of peak operational demand. Boost pump50 also provides a downstream pressure drop through fuel cooler 20, andgenerates an overpressure to limit cavity formation at the inlet to mainfuel pump 56.

Main fuel pump 56 drives flow through fuel-oil-cooler 46 and secondaryheat exchanger 48. Fuel-oil-cooler 46 accepts heat from oil heat load58, including rotor bearings and other elements of a combustion turbineor turbofan engine (e.g., the main engines for a jet aircraft).Alternatively, oil heat load 58 represents a gearbox or otherdifferential rotation system for a turboprop, turboshaft or gearedturbofan engine. Secondary heat-exchanger 48 comprises a fuel-air heatexchanger to cool the compressed air flow from integrated power unit 22,and to pre-heat the fuel before combustion in burner 60.

Valve 62 regulates the recirculation of fuel flow back through RTTcooler 44 to fuel tank 42. As opposed to secondary heat-exchanger 48,which raises the fuel temperature by exchanging heat with compressed airflow from integrated power unit 22, RTT cooler 44 exchanges heat withthe expanded cooling air flow to reduces fuel temperatures for storagein fuel tank 42.

Integrated power unit 22 includes an air-cycle machine with compressor64 and turbine 66, or an auxiliary power unit (APU) comprisingcompressor 64 and turbine 66 in flow series with a combustor or burner.In some embodiments, generator 68 is rotationally coupled to turbine 64and compressor 66, for example using a coaxial shaft and clutchmechanism to swap generator 68 in and out during ground operations, orbased on flight conditions and real-time electrical demand.

Source 70 of bleed air comprises a compressed air supply such as afirst-stage compressor bleed or fan air bleed from the main engine, or athird-stream air source such as an independently modulated bleed flowfrom a downstream compressor section. Alternatively, bleed air isprovided by a ram air intake. In further embodiments, source 70comprises a static inlet for use during ground operations, or acompressed air supply generated by an APU.

Incoming air is compressed and heated by compressor 64, then cooled byheat transfer to fuel circulation system 18 in secondary (fuel-air) heatexchanger 48. In some embodiments, a primary heat exchanger may also beincluded, typically upstream of compressor 64.

The compressed air exchanges heat with the downstream cooling air flowin regenerator (air-air heat exchanger) 72, then expands in turbine 66to produce a low temperature, relatively low-pressure cooling air flowfor thermal management system 10. In air-cycle machine embodiments,there is an overall pressure drop from source 70 to the outlet ofexpansion turbine 66 or energy input from a motor/generator, providingthe energy required to turn compressor 64 and generator 68.

Depending on embodiment, cooling air from integrated power unit 22 maybe mixed with additional cooling fluid from air source 74. Air source 74includes an additional fan or compressor bleed air supply, a ram airintake or a third-stream compressed air source providing a supply ofrelatively cool compressed air. Valve 76 regulates or switches thesource between integrated power unit 22 and air source 74, depending onflight conditions, ambient pressure and temperature, and coolingdemands.

Downstream of mixer valve 76, the cooling air flow exchanges heat withvapor cycle system 12, cooling system 16 and fuel circulation system 18.Generally, temperatures increase in the downstream direction, as heat istransferred to the cooling air from different components of thermalmanagement system 10. The order of the flow series thus depends on theindividual cooling needs of each component, as well as the temperaturedifferential and corresponding entropy and efficiency considerations.

There is an advantage in using the hottest available sink of thermalenergy, as compared to the source temperature; that is, with smalltemperature differential ΔT, because this allows more heat to berejected from the system. To promote rapid heat transfer, however,larger differentials are desired, because the heat transfer (Q) isproportional to temperature differential ΔT. That is,

Q=hAΔT,  [1]

where h is the heat transfer coefficient and A is the heat transfersurface area.

Thermal management also depends on other critical design factorsincluding condensation points and other phase transition temperatures,thermal loading, and environmental (ambient) temperatures and pressures,as compared to the desired cabin and cargo bay conditions across a fullrange of different flight conditions, and the operating temperatureranges for heat loads including galley chillers, avionics, radar systemsand power electronics. Thermal management thus requires constanttradeoffs among different air, fuel, oil, cooling system and VCScomponents, presenting an almost unlimited number of possible systemconfigurations and corresponding design choices, and making the netresults of any particular change or modification difficult to predict.

In the embodiment of FIG. 1, cooling flow passes first through heatexchanger 38 of cooling system 16. This provides the coldest availablecooling fluid at relatively high ΔT, to provide rapid chilling oflow-temperature galley and cabin air systems, and for flight control andother mission-critical systems including radar, avionics and powerelectronics.

Within vapor cycle system 12, cooling air flows through condenser 26first, in order to effect a vapor/liquid phase transition at atemperature at or below the condensation point. De-superheater 14 isdownstream of condenser 26 in the cooling flow series, so that thede-superheat temperature is above the condensation temperature. Thesuperheated vapor phase is hotter than the condensate, so heat can betransferred at higher temperature (lower ΔT), thereby reserving thecooler air for heat sources that required a lower temperature sink.

As shown in FIG. 1, the cooling air flow also exchanges heat with fuelcirculation system 18, reducing the fuel temperature in fuel cooler 20.This puts vapor cycle system 12 in thermal contact with fuel circulationsystem 18, via the cooling air flow over condenser 26, fuel cooler 20and de-superheater 14. Downstream of vapor cycle system 12, cooling airalso passes through regenerator (air-air heat exchanger) 72 and RTTcooler 44, as described above.

Depending on embodiment, downstream air may be used to cool thrustnozzle 78, or other main engine components such as blade or vaneairfoils for the compressor and turbine section, or hot components ofafterburner assembly 54 and burner 60. After core engine or nozzlecooling, cooling air is typically vented to the outside atmosphere.

FIG. 2 is a schematic illustration of vapor cycle system 12 for thermalmanagement system 10, with air-cooled de-superheater 14 and condenser26. Cooling system 16 is shown in generic form, exchanging thermalenergy with evaporator 30 of vapor cycle system 12. Integrated powerunit 22 utilizes an air cycle machine or an APU to produce aregenerating cooling air flow from one or more sources of compressedair, as described above with respect to FIG. 1. Fuel system 18 is alsoshown in generic form, and thrust nozzle 78 is replaced by a genericcooling load 80.

As shown in FIG. 2, de-superheater 14 is located between compressor 24and condenser 26 in the vapor cycle flow series, and between fuel cooler20 and regenerator 72 in the cooling air flow series. In particular,de-superheater 14 is downstream of condenser 26, where the air ishotter, so the de-superheat temperature is higher than the condensationtemperature. Condenser 26 is located downstream of cooling system 16, sothe cooling air temperature is higher at condenser 26 than coolingsystem 16.

Air-cooled de-superheater 14 include a vapor-air heat exchanger to coolthe superheated VCS fluid by exchanging heat with the cooling air flow,lowering the superheated VCS fluid temperature before entering condenser26. The air temperature is increased downstream of condenser 26,reducing ΔT (and the change in entropy) at de-superheater 14. Thisincreases efficiency and raises the system coefficient of performance,as described above.

De-superheater 14 exchanges thermal energy with fuel circulation system18 via the cooling air flow through fuel cooler (air-fuel heatexchanger) 20, in flow series between condenser 26 and de-superheater14. In particular, fuel cooler 20 raises the cooling air temperatureabove the condensation point, at which condenser 26 operates.De-superheater 14 exchanges additional thermal energy with the fuel flowvia downstream components of fuel circulation system 18, as shown FIG.1, above.

Condenser 26 is also air cooled, and is located between cooling system16 and fuel cooler 20 in the cooling air flow series. Condenser 26exchanges additional thermal energy with the cooling air flow throughthe cycling of VCS refrigerant through economizer 28, evaporator 30,compressor 24 and back to de-superheater 14, where de-superheater 14 islocated downstream of fuel cooler 20 in the cooling air flow. Condenser26 exchanges thermal energy with fuel circulation system 18 via air flowover fuel cooler 20, and via downstream components as described abovefor de-superheater 14.

FIG. 3 is a schematic illustration of vapor cycle system 12 for thermalmanagement system 10, with fuel-cooled de-superheater 14. In thisembodiment, de-superheater 14 is located between compressor 24 andcondenser 26 in the vapor cycle flow series, and between secondary(fuel-air) heat exchanger 48 and RTT cooler 44 in the fuel flow series.

Fuel-cooled de-superheater 14 comprises a fuel-vapor heat exchanger tocool the superheated VCS fluid by direct heat exchange with the fuelflow, lowering the superheated refrigerant temperature before enteringcondenser 26. The fuel temperature is increased downstream of secondaryheat exchanger 48, lowering ΔT for de-superheater 14 and reducing thechange in entropy to improve the coefficient of performance and overalloperating efficiency, as described above.

Condenser 26 is also fuel-cooled, and is located downstream of fuelcooler 20 and upstream of boost pump 50 in the fuel flow series. In thisembodiment, condenser 26 condenses the cooled, superheated VCS fluid bydirect heat exchange with the fuel flow. Bypass valve 82 regulates therelative fuel flow through fuel tank 42 and fuel cooler 20, maintainingthe fuel temperature below the condensation point to encourage avapor/liquid phase transition in condenser 26.

While this invention has been described with reference to exemplaryembodiments, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the spirit and scope of theinvention. In addition, modifications may be made to adapt a particularsituation or material to the teachings of the invention, withoutdeparting from the essential scope thereof. Therefore, the invention isnot limited to the particular embodiments disclosed herein, but includesall embodiments falling within the scope of the appended claims.

1. A vapor cycle system comprising: a compressor for compressing arefrigerant to form a superheated vapor; a de-superheater for coolingthe superheated vapor to form a reduced temperature vapor by exchangingheat with a cooling fluid flow; a condenser for condensing the reducedtemperature vapor to form a condensed liquid by exchanging heat with thecooling fluid flow; and an evaporator for evaporating the condensedliquid; wherein the de-superheater is located downstream of thecondenser in the cooling fluid flow, such that a temperature of thecooling fluid flow is higher at the de-superheater than at thecondenser.
 2. The system of claim 1, wherein the cooling fluid flowcomprises an air flow.
 3. The system of claim 2, further comprising anair cycle system for generating the air flow from at least one sourceselected from the group consisting of a ram air supply, a bleed airsupply, or a third-stream air supply from an aircraft engine.
 4. Thesystem of claim 2, wherein the de-superheater comprises an air-vaporheat exchanger to cool the superheated vapor by heat exchange with theair flow.
 5. The system of claim 2, further comprising a heat loadlocated between the condenser and the de-superheater in the cooling airflow; wherein the heat load comprises a heat exchanger to cool a fluidby heat exchange with the air flow.
 6. The system of claim 1, whereinthe cooling fluid flow comprises a fuel flow.
 7. The system of claim 6,wherein the de-superheater comprises a fuel-vapor heat exchanger to coolthe superheated vapor by direct heat exchange with the fuel flow.
 8. Thesystem of claim 1, further comprising a cooling loop for exchanging heatbetween a heat load and the evaporator.
 9. The system of claim 8,wherein the cooling loop comprises a heat exchanger located upstream ofthe condenser in the cooling fluid flow, such that the temperature ofthe cooling fluid flow is lower at the heat exchanger than at thecondenser.
 10. The system of claim 9, wherein the heat load comprises atleast one selected from the group consisting of a passenger cabin, acargo bay, a galley, an avionics system, a radar system and powerelectronics for actuating a flight control surface.
 11. A thermalmanagement system for an aircraft, the system comprising: a coolingloop, wherein the cooling loop circulates coolant inside the aircraft;and a vapor cycle unit, wherein the vapor cycle unit operates on a fluidto cool the coolant, the vapor cycle unit comprising: a compressor,configured to compress the fluid to a superheated vapor phase; ade-superheater, configured to cool the fluid from the superheated vaporphase to a cooled vapor phase by exchanging heat with a cooling flow; acondenser, configured to condense the fluid from the cooled vapor phaseto a condensed liquid phase by exchanging heat with the cooling flow; anexpansion device, configured to expand the fluid from the condensedliquid phase to an expanded phase; and an evaporator, configured to coolthe coolant by evaporation of the fluid from the expanded phase to thevapor phase; wherein the de-superheater is located downstream of thecondenser in the cooling flow, such that the cooling flow is hotter atthe de-superheater than at the condenser.
 12. The system of claim 11,wherein the cooling flow comprises a fuel flow for the aircraft, andwherein the de-superheater comprises a vapor-fuel heat exchanger forcooling the superheated vapor phase by direct heat exchange with thefuel flow.
 13. The system of claim 11, wherein the cooling flowcomprises a cooling air flow.
 14. The system of claim 13, furthercomprising a fuel cooler located downstream of the condenser andupstream of the de-superheater in the cooling air flow, such that thecooling flow is hotter at the de-superheater than at the fuel cooler.15. The system of claim 13, wherein the cooling loop comprises a heatexchanger located upstream of the condenser in the cooling air flow,such that the cooling air flow is colder at the heat exchanger than atthe condenser.
 16. A method for cooling an aircraft system, the methodcomprising: circulating a coolant through the aircraft system; coolingthe coolant by exchanging heat with an evaporating fluid; compressingthe evaporating fluid to form a superheated vapor; de-superheating thesuperheated vapor by exchanging heat with a cooling flow at ade-superheat temperature; and condensing the superheated vapor byexchanging heat with the cooling flow at a condensation temperature,wherein the condensation temperature is lower than the de-superheattemperature.
 17. The method of claim 16, further comprising generatingthe cooling flow as a fuel flow for the aircraft by pumping fuel from afuel tank.
 18. The method of claim 16, further comprising generating thecooling flow as a stream of cooling air by expansion of a compressed airsource.
 19. The method of claim 16, further comprising cooling thecoolant by exchanging heat with the cooling flow at a temperature lowerthan the condensation temperature.
 20. The method of claim 16, furthercomprising cooling a fuel flow for the aircraft by exchanging heat withthe cooling flow at a temperature higher than the condensationtemperature and lower than the de-superheat temperature.